1. Field of the Invention
The present invention relates in general to a photovoltaic cell and a method of forming the same. More particularly, it relates to such a technology suitable for forming a flexible thin film photovoltaic cell.
2. Description of the Prior Art
Amorphous silicon thin film solar cells have been known as economical solar cells which can be formed on a large area of a glass substrate with a semiconductor laminate film having a thickness as thin as one micron or less. Particularly, when processed in accordance with the laser scribing technique, wide panels of this kind of solar cell can be easily produced with a high productivity.
Thin film solar cells have generally been fabricated by thin film semiconductors formed by vapor phase reactions, evaporation, sputtering, and so forth. Amorphous silicon semiconductors can be deposited on flexible substrates such as plastic films as well as glass substrates. Solar cells formed on such a flexible substrate also possess flexibility so that they can be provided on curved surfaces. Since typical flexible substrates are lightweight, the weight of a solar cell can be substantially reduced. Accordingly, the applicability of the amorphous thin film solar cell is greatly extended.
FIGS. 1(A) to 1(E) are cross sectional views showing a conventional method of forming thin film solar cells. A transparent conductive thin film 22 made of SnO.sub.2 ; ITO, or the like is formed on a glass substrate 21 and patterned by laser scribing to divide the film 22 into a plurality of transparent conductive electrodes as illustrated in FIG. 1(A). An amorphous silicon semiconductor photoelectric conversion layer 23 is formed on the substrate 1 over the transparent conductive electrodes as illustrated in FIG. 1(B). A PIN semiconductor junction is formed within the photoelectric conversion layer 23 for photoelectric conversion. The photoelectric conversion layer 23 is then divided into a plurality of photoelectric conversion regions by laser scribing corresponding to the underlying transparent conductive electrodes as illustrated in FIG. 1(C). The entire structure is coated with a metallic film 24 made of aluminum or chromium which provides a reflective rear surface as illustrated in FIG. 1(D). In the case of a see-through type (transmission type), the reflective metallic film is replaced by a transparent conductive film. The metallic film 24 is then divided into a plurality of rear electrodes by laser scribing corresponding to the underlying photoelectric conversion regions and the transparent conductive electrodes as illustrated in FIG. 1(E) in order to provide a series connection of photoelectric conversion units each consisting of the conductive transparent electrode, the photoelectric conversion region, and the rear electrode. It has been also proposed to form flexible solar cells by laser scribing on flexible substrates utilized in place of the glass substrate in the above method. The term "solar cell" is used to designate a general photoelectric conversion device consisting of a plurality of the photoelectric conversion units connected in series, each of which is a minimum construction capable of generating electric energy converted from optical energy.
There are, however, two serious problems associated with the above conventional technique which make it difficult to form highly integrated solar cells by the use of the laser scribing technique. One of these problems is a general problem which occurs irrespective of the kind of the substrate. Namely, the problem occurs when the rear conductive film 24 is divided into the plurality of the rear electrodes by laser scribing. If the output power of the laser is limited in order not to damage the underlying semiconductor layer 23, the rear electrodes are often electrically shorted since the scribing may have been incomplete to sever the conductive film 24 into separate rear electrodes. On the contrary, if the output power of the laser is sufficiently high in order to ensure the division of the conductive film 24, the underlying semiconductor layer 23 is removed or damaged at the same time so that the yield of the solar cells is substantially decreased.
The other problem occurs when a flexible film is utilized as the substrate in place of the glass substrate. The method as shown in FIGS. 1(A) to 1(E) can be basically utilized on a flexible substrate such as a plastic sheet in the same manner. The process, however, must be carried out at such low temperatures as not to damage the flexible film. Several types of plastics for industrial usage and organic resins have been well known in the art. The heat resisting properties of such flexible substrates are very poor and in general they are deteriorated at temperatures no lower than 100.degree..
The transparent conductive film 22 is severed by laser scribing also when the substrate 21 is made of a flexible film. Since the transparent conductive film must be completely separated, the flexible substrate 21 is necessarily exposed to laser light and therefore damaged at the resulting high temperatures. FIG. 2 schematically illustrates the damaged portion of the flexible substrate 21. The damage due to the high temperature tends to be spread around the exposed area and causes the overlying transparent conductive film 22 to lift off from the flexible substrate as illustrated in FIG. 2. The conductive film which is lifted off often forms separate flakes. The sizes of the damaged areas and the flakes easily reach several micrometers while the thickness of the photoelectric conversion layer is no thicker than 1 micrometer. As a result, a short current path is formed which bridges the transparent conductive electrode and the metallic electrode at a high probability. This is inevitable since there has been found no flexible material suitable for use in manufacture of solar cells and having a high heat resistance. Of course, although not so significant, the above problem is also the case when flexible films other than plastic films are utilized as the substrate and when a glass substrate is utilized as the substrate.